Visual acuity is tested by presenting to client, i.e. the person who is being tested, sets of symbols of various sizes, i.e. optotypes, at a standard distance that approximates infinity in the way that the eye attempts to perceive objects and having the client indicate the smallest optotype that she/he can identify.
The need for standardized vision tests was recognized in the mid eighteen hundreds and shortly thereafter, in 1862, Hermann Snellen developed his well-known and probably the most used of all the eye charts that have been proposed since that time. The optotypes on the Snellen chart are letters that are arranged in a series of rows. The optotypes in each row are of the same size and they are arranged on the page such that, at the standard distance of six meters, the visual angle subtended by the distance between the horizontal and verticals elements that make up each letter is one arc minute and the visual angle subtended by the optotype is five arc minutes. The chart is arranged such that the angle between elements in the letters in successively higher rows increases.
Over the years many other types of chart have been developed. The basic principle of the test for visual acuity has remained the same since the time of Snellen with the major differences related to different forms of optotypes developed, for example, for use with children or clients not familiar with Latin letters.
The results (scores) of visual acuity test are normally recorded as the ratio of the distance between the client's eye and the optotypes divided by the distance at which the smallest optotypes that can be identified by the client could be seen by a person with “standard” vision. Thus a score of 6/6 means that the client can identify an optotype at 6 meters that the standard person can identify at 6 meters, i.e. the client has standard visual acuity. A score of 6/60 means that the client can identify at 6 meters what person having standard visual acuity can identify at 60 meters, i.e. the client has very poor visual acuity, and a score of 6/4 means that the client has much better visual acuity than the standard person.
Many different methods of displaying the optotypes to the client have been developed over the years. The most common method is by means of a chart comprised of rows of optotypes printed in black on a white background that is hung on a wall. Another common method is to print the optotypes on a transparent substrate that is backlighted to improve contrast. In other methods the entire chart or individual optotypes are reproduced on slides that are optically projected on a wall or screen the required distance from the client. Much more sophisticated display methods are described, for example in U.S. Pat. No. 4,239,351 and US 2011/0116047.
U.S. Pat. No. 4,239,351 describes an apparatus for testing visual acuity based on two electro-optic display devices to be viewed by a patient and an examiner respectively. Typically the apparatus is used for testing by presenting a single optotype to the patient at a size below which he is able to identify it and then rapidly zooming the optotype to a larger size until the patient can identify it. The examiner then sequentially presents rows of optotypes of increasingly smaller size until the patient can identify all of them. According to the method in this patent the zoom function is used only to find an approximate starting point for carrying out the visual acuity test using a row by row presentation of the optotypes.
US 2011/0116047 describes a method of visual evaluation or training based on an apparatus that displays symbols on a display device and electronically and optically changes the parameters, including the size, of the symbols displayed. The display device is a hand held device and the evaluation or training session is controlled and results recorded either by software installed on the hand held device or remotely on a server connected to the hand held device via the Internet or a cellular phone network. Also described is a “staircase” method of testing visual acuity in which the display and client are much closer together than in the usual methods. In this case the display device can be stationary, e.g. a desktop computer screen, or a handheld device. The client is required to respond to a displayed series of symbols in which the physical size of the symbols is sequentially reduced in a step-like fashion. The response of the client can be oral or by activating an input device such as a keyboard. The response depends on the nature of the symbol and can be, for example, the name of a letter or, the shape or orientation. When the client can no longer correctly identify the symbol displayed on the screen the system can automatically calculate a score for the test.
Because of the sequential manner in which the optotypes are displayed or read from the charts methods of testing for visual acuity are relatively time consuming. The results are very subjective since they are dependent on the responses and in some cases the reaction time of the client. In some cases the results are influenced by guesswork or by memorization from previous exposure to the symbols and or the sequence in which they are displayed.
One way to reduce these effects and to increase the accuracy of the results is to repeat the test one or more times but this increases the required time.
It is a purpose of the present invention to provide a method for testing the visual acuity of a person that minimizes the drawbacks of the prior art.
Further purposes and advantages of this invention will appear as the description proceeds.